I'm not familiar with the Imperial grill but a char-grill is a char-grill is a char-grill. What you are looking for is a high BTU (British Thermal Unit). This indicates the amount of power, or heat, the unit can chuck out, so the higher the number the hotter the grill, the quicker the recovery time and the more produce you can cook on it at any one time without the loss of heat. The other important considerations are; How robust is the unit How easy is it to clean Does it have a grease trap Does it have independent heat controls for different sections (i.e. can you turn half of the char-grill off during quiter sessions) What I do know of Imperials gear is that it is very robust (being American) and they do tend to be of good quality. Hope this helps,
Fergo
Lava rock can make a grill more difficult to clean so some manufacturers use heat transfer radiants instead. The igniting of fat and flaring is what gives food a chargrilled taste rather than the lava rock itself. Check out last month's grills feature on Caterersearch:
http://www.caterersearch.com/Articles/2009/04/24/327308/bbqs-and-grills-the-outsiders.html